Metal organic chemical vapor deposition (MOCVD) has been used in epitaxial growth of group III nitride semiconductor thin films for the reason that the MOCVD tends to achieve high productivity. Although conventional group III nitride semiconductor thin films grown by the MOCVD would be mostly of low quality having threading dislocation densities in a range from the upper half of the 109 cm−2 order to the 1010 cm−2 order, high-quality single-crystal films thereof have been becoming available with advances in technical developments in recent years. For example, in blue LEDs currently available in the market, the threading dislocation density has been successfully reduced to about 1×109 cm−2 in the case of crystal growth of a group III nitride semiconductor thin film in the thickness of about 5 to 10 μm on a sapphire substrate having a flat surface. This threading dislocation density is significantly larger value than those of other compound semiconductor devices. Nevertheless, this value represents quite fine crystallinity as the group III nitride semiconductor thin film, which is formed on the sapphire substrate having the flat surface and is used for a blue LED.
Considering further improvements in device properties in the future, a group III nitride semiconductor thin film is desired to achieve a threading dislocation density of about 5×108 cm−2, or is even more desired to have a threading dislocation density of about 1×108 cm−2. However, on the sapphire substrate having the flat surface, it is difficult to reduce the threading dislocation density to about 5×108 cm−2, and it is even more difficult to further reduce the threading dislocation density to about 1×108 cm−2. In this regard, an approach to reduce the threading dislocation density, for example, by using any of a sapphire substrate provided with irregularities on its surface and a silicon carbide substrate is currently under study. However, the use of the substrates mentioned above is likely to cause a problem of an increase in cost of the substrates.
In the meantime, another method is currently under study which obtains a high-quality group III nitride semiconductor thin film by forming a buffer layer made of an AlN film and deposited by sputtering on a sapphire substrate, and forming a foundation layer made of group III nitride semiconductor thereon by the MOCVD (Patent Document 1, for example). Patent Document 1 discloses that an oxygen content of less than 1% in the buffer layer made of the AlN film improves lattice match between the sapphire substrate and the buffer layer, improves orientation of the buffer layer, and consequently improves crystallinity of the group III nitride semiconductor thin film formed on the buffer layer. Patent Document 1 also discloses that, in order to form a foundation layer (which is group III nitride semiconductor containing Ga according to Patent Document 1) with fine crystallinity on the buffer layer made of the AlN film that contains less than 1% of oxygen, the film thickness of the foundation layer is preferably set in a range from 0.1 to 8 μm, or preferably set in a range from 0.1 to 2 μm from the viewpoint of its productivity.